1. Field of the Invention
The present invention relates to a tungsten oxide photocatalyst, particularly to a photocatalyst which shows high photocatalytic activity even under the environment where visible light is irradiated while ultraviolet light is not irradiated, and also can completely decompose organic materials such as acetic acid and acetaldehyde.
2. Description of the Related Art
When a semiconductor is irradiated with light having energy greater than the bandgap, electrons of the valence band are excited into the conduction band to form holes in the valence band and electrons in the conduction band. These electrons and holes respectively have strong oxidizing power and reducing power and exert an oxidation-reduction reaction on molecular species contacted with the semiconductor. Such a reaction is referred to as a photocatalytic reaction and such a semiconductor is referred to as a photocatalyst.
It is considered that, in a decomposition reaction of an organic material through a photocatalyst, holes formed in the valence band directly cause the oxidative decomposition of the organic material or holes cause oxidation of water, and reactive oxygen species formed therefrom cause oxidative decomposition of the organic material. It is also considered that electrons formed in the conduction band causes reduction of oxygen and reactive oxygen species formed therefrom cause oxidative decomposition of the organic material.
As the photocatalyst, a photocatalyst using tungsten oxide particles is known. The tungsten oxide particles can absorb visible light making up a majority of light in the indoor space, and is remarked as a visible light responsive photocatalyst. However, when the photocatalyst using tungsten oxide particles is irradiated with visible light, holes and electrons are respectively formed in the valence band and the conduction band through photoexcitation. Since the conduction band is located at the lower position than that of an oxidation-reduction level of oxygen, oxygen cannot be reduced by electrons excited to the conduction band and a sufficient amount of reactive oxygen species are not formed. Therefore, tungsten oxide did not show high photocatalytic activity under the environment where visible light is irradiated while ultraviolet light is not irradiated.
Japanese Unexamined Patent Publication (Kokai) No. 2001-038217 (Japanese Patent No. 3887510)) discloses a photocatalyst capable of efficiently decomposing a decomposition intermediate of acetaldehyde by forming a film made of a noble metal such as Pt on a tungsten oxide film. However, there was a problem that sufficient photocatalytic activity cannot be obtained and also an expensive apparatus is required to prepare the photocatalyst.